1. Field of the Invention
The present invention relates to a flat lamp and a method of driving the same, and more particularly, to a flat lamp used in a light source unit for a flat display, for example, a back light unit for a liquid crystal display (LCD), and a method of driving the same.
2. Description of the Related Art
An LCD is a representative light receiving flat display, and a plasma display panel (PDP) is a representative self-luminescent flat display. A PDP has advantages such possibility of being used in a large screen and a memorization characteristic, but it is difficult to be manufactured in a small size. Accordingly, a PDP is usually used for a large-screen TV. An LCD is equal to or better than a PDP in performance. Accordingly, an LCD is used for most small or middle-sized displays.
An LCD includes a back light unit (BLU) as a light source unit for uniformly illuminating light on the entire surface of a liquid crystal panel. The BLU includes a light source, and the structure of the BLU changes depending on the type of the light source.
Cold cathode fluorescent lamps are widely used as the light sources in BLUs. However, the use of cold cathode fluorescent lamps has gradually decreased due to unsatisfactory luminance, uniformity, and environmental affinity. Surface discharging type or facing surfaces discharging type flat lamps have been developed as light sources replacing cold cathode fluorescent lamps and have already been used in some flat display products.
In flat lamps, other discharging gas than mercury, for example, xenon (Xe), is used. Accordingly, flat lamps are better than cold cathode fluorescent lamps in terms of environmental affinity. In addition, a flat lamp is installed in the back of a flat display panel, e.g., a liquid crystal panel, in parallel with the liquid crystal panel. The size of the flat lamp is usually the same as the size of the liquid crystal panel. Accordingly, when a flat lamp is used as a light source of a flat display, light loss is reduced, and therefore, luminance increases. Moreover, since a flat lamp radiates light from the entire surface at a uniform intensity, light is radiated on the entire surface of a display panel, e.g., a liquid crystal panel, of a flat display, thereby increasing the uniformity of the flat display.
Various types of flat lamps have been developed. FIGS. 1 and 2 are cross-sections of examples of a conventional flat lamp.
Referring to FIG. 1, a conventional flat lamp includes a front panel 10, a rear panel 20, and spacers 22, provided in a discharge space 24 between the front panel 10 and the rear panel 20, for supporting the front panel 10 and the rear panel 20. The front panel 10 includes a first glass substrate 10a and a first fluorescent layer 10b formed on the back surface of the first glass substrate 10a. The rear panel 20 includes a second glass substrate 20a, a dielectric layer 20b and a second fluorescent layer 20c which are sequentially formed on the second glass substrate 20a, and a plurality of electrodes 20d and 20e which are arranged in a striped pattern between the second glass substrate 20a and the dielectric layer 20b. An electrode 20d and an electrode 20e form an electrode pair, and a plurality of electrode pairs exist between the second glass substrate 20a and the dielectric layer 20b. The plurality of electrode pairs are separated from one another by a predetermined distance d1, which may be greater than a distance d2 between two electrodes in each electrode pair (d1>d2).
When a driving voltage higher than a discharge starting voltage is applied to the electrodes 20d and 20e of the rear panel 20, a discharge occurs between two electrodes in each electrode pair. Due to these discharges, high-temperature electrons are produced in the discharge space 24. The high-temperature electrons excite a neutral discharge gas, e.g., Xe gas, in the discharge space 24. When the excited discharge gas returns to a base state, ultraviolet rays are radiated from the excited discharge gas. The radiated ultraviolet rays excite a fluorescent material of the first and second fluorescent layers 10b and 20c. When the excited fluorescent material returns to the base state, visual light is radiated from the fluorescent material. As a result, visual light is radiated from the first and second fluorescent layers 10b and 20c, and then the radiated visual light is transmitted outside the flat lamp through the first glass substrate 10a. 
Another conventional flat lamp having a different structure from the flat lamp shown in FIG. 1 will be described with reference to FIG. 2. In the flat lamp shown in FIG. 2, a plurality of igniters 30a are provided on the back surface of a front glass substrate 30. Each igniter 30a includes two wires, which protrude from the back surface of the front glass substrate 30 and are spaced a predetermined distance apart. The igniter 30a is used to trigger a discharge. A plurality of electrodes 40a, 40b, and 40c are repeatedly arranged on a rear glass substrate 40, facing the front glass substrate 30, at predetermined distances, which are greater than the distance between two wires in each igniter 30a, such that two auxiliary electrodes 40b and 40c are respectively arranged at opposite sides of a main electrode 40a in parallel. A plurality of tips P are alternately formed along each main electrode 40a at predetermined distances. The tips P face opposite directions.
When a discharge is triggered by the igniter 30a, plasma is formed between the front glass substrate 30 and the rear glass substrate 40, and therefore, charged particles are produced between the front glass substrate 30 and the rear glass substrate 40. The discharge triggered by the igniter 30a is sustained due to a surface discharge among the electrodes 40a, 40b, and 40c. Due to the charged particles, the surface discharge occurs at a low voltage. A surface discharge area among the electrodes 40a, 40b, and 40c includes a plurality of microscopic surface discharge areas.
More specifically, a single microscopic surface discharge area 40e is formed between one of the tips P formed along the main electrode 40a and the auxiliary electrode 40b or 40c facing the tip P. Accordingly, as many microscopic surface discharge areas 40e as the number of tips P formed along the main electrode 40a are formed among the main electrode 40a and the auxiliary electrodes 40b and 40c. A surface discharge area among the main electrode 40a and the auxiliary electrodes 40b and 40c is the sum of these microscopic surface discharge areas 40e. 
Accordingly, when the conventional flat lamp shown in FIG. 2 is used, a discharge can be prevented from being concentrated on particular portions of adjacent electrodes, and a reliable discharge can be provided because a surface discharge area includes many microscopic discharge areas 40e. 
However, each microscopic discharge area 40e becomes wider from a peak, i.e., a tip P, to an auxiliary electrode 40b or 40c facing the tip P, at an angle much smaller than 180°. Accordingly, an area, in which a microscopic discharge does not occur, may exist between tips P although it is very small. In addition, the distance between the main electrode 40a and the auxiliary electrodes 40b and 40c is wide. Accordingly, luminance and uniformity are unavoidably decreased.
In the meantime, when visual light is obtained using a gas discharge as in a flat lamp, a luminescence efficiency is calculated using the following formula:
  η  =                    π        ⁢                                  ⁢        SB            W        =                            π          ⁢                                          ⁢          SB                VI            .      
Here, η denotes a luminescence efficiency, π denotes the ratio of the circumference of a circle to its diameter, S denotes a display area, B denotes a luminance, W denotes a power consumption, and V and I denote a voltage and a current, respectively.
Theoretically, luminescence efficiency can be increased by increasing the pressure of a discharge gas and a distance between electrodes. However, when the pressure and the distance are increased, a discharge voltage increases, and thus a driving integrated chip (IC) having a high withstand voltage is required. As a result, the price of products is increased. Conversely, if the pressure of a discharge gas and the distance between electrodes are decreased in order to avoid these problems, luminance and luminescent efficiency are decreased, which is worse than an increase in the price.
Therefore, in most of the conventional flat lamps, the pressure of a discharge gas is high, and the distance between electrodes is wide, and thus a discharge voltage is high. One kind of these lamps is shown in FIG. 1.
In the case of a flat lamp as shown in FIG. 2, since an igniter is provided on the back surface of a front glass substrate, a discharge voltage is lower than the flat lamp shown in FIG. 1. However, as described above, luminance is decreased. Since the luminance B is proportional to the luminescence efficiency η, as shown in the above formula, a discharge voltage can be lowered, but luminescence efficiency is degraded in the flat lamp shown in FIG. 2.